Asserts that the {@link ChromeWindow} is open or that the {@link
browser.Context} has a content browser attached.

When passed in a {@link ChromeContext} this is equivalent to
testing that the associated <code>window</code> global is open,
and when given {@link browser.Context} it will test that the content
frame, represented by <code>&lt;xul:browser&gt;</code>, is
connected.